TitleHorizontal distribution and population dynamics of thedominant mysid Hyperacanthomysis longirostris along atemperate macrotidal estuary (Chikugo River estuary, Japan)

Author(s) Suzuki, Keita W.; Nakayama, Kouji; Tanaka, Masaru

Citation Estuarine, Coastal and Shelf Science (2009), 83(4): 516-528

Issue Date 2009-08

URL http://hdl.handle.net/2433/80150

Right

c 2009 Elsevier Ltd. All rights reserved.; This is not thepublished version. Please cite only the published version. この論文は出版社版でありません。引用の際には出版社版をご確認ご利用ください。

Type Journal Article

Textversion author

Kyoto University

Horizontal distribution and population dynamics of the dominant mysid

Hyperacanthomysis longirostris along a temperate macrotidal estuary (Chikugo River

estuary, Japan)

Keita W. Suzukia,*, Kouji Nakayamab, Masaru Tanakab

aDivision of Applied Biosciences, Graduate School of Agriculture, Kyoto University,

Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

bField Science Education and Research Center, Kyoto University, Oiwake-cho,

Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

*Corresponding author.

Tel: 81-75-753-6222. Fax: 81-75-753-6229

E-mail address: keita@kais.kyoto-u.ac.jp

Postal address: Laboratory of Marine Stock-Enhancement Biology, Division of Applied

Biosciences, Graduate School of Agriculture, Kyoto University, Oiwake-cho,

Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan

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Abstract

The estuarine turbidity maximum (ETM) that develops in the lower salinity

areas of macrotidal estuaries has been considered as an important nursery for many fish

species. Mysids are one of the dominant organisms in the ETM, serving as a key food

source for juvenile fish. To investigate the horizontal distribution and population

dynamics of dominant mysids in relation to the fluctuation of physical conditions

(temperature, salinity, turbidity, and freshwater discharge), we conducted monthly

sampling (hauls of a ring net in the surface water) along the macrotidal Chikugo River

estuary in Japan from May 2005 to December 2006. Hyperacanthomysis longirostris

was the dominant mysid in the estuary, usually showing peaks of density and biomass in

or close to the ETM (salinity 1-10). In addition, intra-specific differences (life-cycle

stage, sex, and size) in horizontal distribution were found along the estuary. Larger

males and females, particularly gravid females, were distributed upstream from the

center of distribution where juveniles were overwhelmingly dominant. Juveniles

increased in size toward the sea in marked contrast with males and females. The

findings suggest a possible system of population maintenance within the estuary; gravid

females release juveniles in the upper estuary, juveniles grow during downstream

transport, young males and females mature during the upstream migration. Density

and biomass were primarily controlled by seasonal changes of temperature, being high

at intermediate temperatures (ca. 15-25 ºC in late spring and fall) and being low at the

extreme temperatures (ca. 10 ºC in midwinter and 30 ºC in midsummer). High density

(up to 666 ind. m-3) and biomass (up to 168 mg dry weight m-3) of H. longirostris were

considered to be comparable with those of copepods in the estuary.

Keywords: mysid; Hyperacanthomysis longirostris; distribution; population dynamics;

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salinity; estuarine turbidity maximum; Japan; Ariake Sea; Chikugo River estuary

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1. Introduction

Mysids are especially abundant in coastal and estuarine shallow waters, where

they occupy an important position between lower and higher trophic levels (Mauchline,

1980). It is well established that juveniles of various fish species utilize mysid

production in coastal and estuarine nursery grounds (Hostens and Mees, 1999; Feyrer et

al., 2003; Nemerson and Able, 2004). The distribution and population dynamics of

mysids are, therefore, significant in consideration of coastal fish production. In this

context, several studies on the ecology of mysids have been conducted in estuaries,

emphasizing spatial distribution (e.g. Hulburt, 1957; Baldό et al., 2001) and seasonal

production (e.g. Wooldridge, 1986; Mees et al., 1994). However, the mechanism that

mysid species use to maintain a population at a certain location within an estuary, where

physical conditions drastically change in various spatial and temporal scales, remains

unclear. More specifically, extensive research on the different horizontal distributions

between sexes and/or life-cycle stages within an estuary has been carried out (Hulburt,

1957; Mauchline, 1970; Orsi, 1986; Hough and Naylor, 1992; Moffat and Jones, 1993;

Baldό et al., 2001); however, a comprehensive explanation in terms of population

maintenance remains obscure, although mechanisms of position maintenance for

respective sexes and life-cycle stages were sometimes discussed in the previous studies.

The Chikugo River estuary, which is located in the innermost part of the Ariake

Sea, southwestern Japan (Fig. 1), is a macrotidal estuary with a well-developed

estuarine turbidity maximum (ETM). This is one of the most productive fishery areas

in Japan, holding a large number of semi-endemic species (i.e. species found only in the

Ariake Sea within Japan; reviewed in Sato and Takita, 2000) from several taxonomic

groups, such as fish (e.g. Neosalanx reganius, Takita, 1996; Trachidermus fasciatus,

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Cao et al., in press) and crustaceans (e.g. Sinocalanus sinensis, Hiromi and Ueda, 1987;

Tortanus derjugini, Ohtsuka et al., 1995; Acartia ohtsukai, Ueda and Bucklin, 2006).

The mysid Hyperacanthomysis longirostris (Fukuoka and Murano, 2000) is also

considered as a semi-endemic species (Chihara and Murano, 1997), found particularly

in lower salinities in rivers flowing into the innermost part of the Ariake Sea (Ikematsu,

1963; Suzuki et al., 2008b). Semi-endemic species of the Ariake Sea are considered as

continental relicts that presently survive in restricted areas in Japan, being derived from

the isolation of the Japanese archipelago from the Eurasian continent by marine

transgressions during geological history (Sato and Takita, 2000).

So far copepods have been well studied as an important food for larval and

early juvenile fish in the ETM of the Ariake Sea (e.g. Hibino et al., 1999; Shoji et al.,

2006; Islam et al., 2006a, 2006b). In addition, several species of mysids recorded from

the Ariake Sea (Ikematsu, 1963) have often been found in stomachs of larger juvenile

fish of semi-endemic (Kuno and Takita, 1997; Mizutani and Matsui, 2006) and

non-semi-endemic species (Takita and Intong, 1991; Suzuki et al., 2008a). For

instance, Japanese temperate bass Lateolabrax japonicus juveniles exclusively feed on

mysids in the ETM, after they ontogenetically change prey categories from copepods

(Suzuki et al., 2008a). Although ecological information on mysids is important to

understand the ecosystem supporting fish production, particularly semi-endemic fish

populations in the Ariake Sea, available data from this sea area is restricted to general

descriptions of several mysid species (Ikematsu, 1963; Suzuki et al., 2008b).

The objective of the present study is to investigate the horizontal distribution

and population dynamics of dominant mysid species along the Chikugo River estuary,

focusing on relationships with physical conditions (temperature, salinity, turbidity, and

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freshwater discharge). To characterize seasonal and annual changes, we conducted

monthly sampling over the entire tidal area of the estuary for 20 months. We identified

biological properties (life-cycle stage, sex, and size) of the dominant mysid

Hyperacanthomysis longirostris at each station and hypothesized a possible system that

would contribute to the population maintenance by comparison of the properties

between stations, months, and years.

2. Methods

2.1. Study area

Sampling was conducted along the lower reaches and the tidal channel of the

Chikugo River (Fig. 1), the largest river flowing into the Ariake Sea. The Chikugo

River estuary is characterized by large tidal ranges, vast tidal flats, and extremely turbid

water. Since the water column is almost completely mixed during spring tides (tidal

range > 4 m), turbidity exceeds 200 NTU even in the surface water at lower salinities

(Shoji et al., 2006; Suzuki et al., 2007). The ETM is transported back and forth over a

20 km range along the estuary with tidal movements between low and high tides

(Shirota and Tanaka, 1981), although it is usually located 10 to 20 km upstream from

the river mouth at spring high tide (Shoji et al., 2006; Suzuki et al., 2007). As for

copepods true estuarine species (i.e. species that complete their entire life cycle within

the low salinity areas, Sinocalanus sinensis and Pseudodiaptomus inopinus) are

overwhelmingly dominant in or close to the ETM, whereas common coastal species (e.g.

Acartia omorii, Oithona davisae, and Paracalanus parvus) are distributed in the lower

estuary (Hibino et al., 1999; Islam et al., 2006a). Among mysids, Hyperacanthomysis

longirostris is abundant in the upper estuary (Suzuki et al., 2008b), whereas other

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species (e.g. Neomysis japonica and Neomysis awatschensis) are also recorded in the

lower estuary (Ikematsu, 1963).

Seven stations (R1-R7; Fig. 1) were set up along the lower reaches of the river,

ranging from the river mouth to the upper limit of the tidal area, where the Chikugo

Weir is located (23 km upstream). Three stations (E1-E3; Fig. 1) were set up along the

tidal channel of the river in such a way that E1 was near the mouth of the river and E3

was at the edge of the tidal flat (9 km offshore). The freshwater discharge is

continuously monitored and data uploaded to the web site

(http://www.qsr.mlit.go.jp/chikugo/) by the Chikugogawa River Office at Senoshita (3

km upstream from the weir).

2.2. Field sampling

Ring-net hauls and environmental surveys were conducted monthly at the ten

stations from May 2005 to December 2006. All of the sampling dates (23 May, 24

June, 24 July, 23 August, 19 September, 17 October, 15 November, 15 December 2005,

15 January, 14 February, 16 March, 14 April, 27 May, 30 June, 25 July, 24 August, 22

September, 23 October, 21 November, 22 December 2006) approximately corresponded

with spring tides. A ring net (1.3 m mouth diameter, 1 mm mesh aperture along the 3.5

m cylindrical body and 0.33 mm mesh aperture at the 1.5 m conical end) equipped with

a digital flow meter was towed in the surface water by a boat for 10 min at

approximately 1 m s-1 relative to the tidal flow. The duration of towing was sometimes

reduced to prevent clogging of the net by excessive zooplankton and suspended matter.

Catches from the hauls were preserved onboard in 99 % ethanol. The water volume

filtered through the net was estimated using the flow meter. Temperature, salinity,

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turbidity were measured from the bottom to the surface at 1 or 2 m intervals with an

environmental monitoring system (YSI 650 MDS, YSI, USA). Since the turbidity unit

of the monitoring system was sometimes out of order (on 24 July 2005 and 14 February

2006), turbidity was measured with another monitoring system (Compact-CTD, Alec

Electronics, Japan) after 16 March 2006. Hauls and surveys were started at the

uppermost station (R7) and finished at the lowermost station (E3) within 4-5 h around

high tide in the morning. Owing to strong waves, we could not conduct sampling at

E3 on 30 June 2006.

To confirm the validity of ring-net hauls in the surface water, mysids were

repeatedly collected at three depths (surface, middle, and bottom) with a small conical

ring net (45 cm mouth diameter, 200 cm long, 0.33 mm mesh aperture) equipped with a

digital flow meter on 24 October 2006. A weight and a buoy were symmetrically tied

to the ring of the net. After adjusting the length of the rope between the buoy and the

ring, the net was towed at a certain depth for 3 min at approximately 1 m s-1 relative to

the tidal flow. Temperature, salinity, and turbidity were measured with the

Compact-CTD at intervals of 0.1 m and current velocity was measured with a digital

flow meter (VR-201, KENEK, Japan) at intervals of 1.0 m at each station. We

conducted seven series of sampling between R2 and R5 at intervals of an hour around

high tide in the morning, tracking a water mass of approximately salinity 1. The water

mass was selected because mysids had been abundantly observed on the previous day

(i.e. 23 October 2006).

2.3. Laboratory analysis

Mysid species were identified under a dissecting microscope according to

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references (Chihara and Murano, 1997; Fukuoka and Murano, 2000, 2005), before

being counted quantitatively for each station every month. As for the dominant mysid

Hyperacanthomysis longirostris, approximately 100 individuals were randomly picked

for classification into four categories (gravid females, non-gravid females, males, and

juveniles). On the basis of secondary sexual characteristics, gravid females were

defined as those with eggs or larvae in marsupiums; non-gravid females, those with

empty marsupiums; males, those with lobus masculina at the antennule peduncle;

juveniles, those without secondary sexual characteristics. Additional individuals were

analyzed, when juveniles accounted for more than 90% of the first 100 individuals

analyzed. The carapace length (from the base of the eyestalk to the posterior

mediodorsal margin of the carapace) was measured using an eyepiece graticule in a

dissecting microscope. To determine the relationship between carapace length and dry

weight, 124 individuals (22 gravid females, 22 non-gravid females, 25 males, and 55

juveniles) collected on 23 May 2005 were dried at 60ºC for 12 h after the measurement

of carapace length. The juveniles were pooled (up to 13 individuals) by the carapace

length class of 0.1 mm (0.7-1.4 mm) to obtain sufficient material for precise

measurements of dry weight. After the measurement of dry weight with an electric

balance, the following equation was fitted on the data of carapace length (CL) and dry

weight (DW):

DW = aCLb, (1)

where a and b are constants. At each station where Hyperacanthomysis

longirostris was collected, the mean dry weight (mg ind.-1) was calculated using both

equation (1) and composition of carapace length, to convert density (ind. m-3) into

biomass (mg m-3). Although there is a possibility that some body contents other than

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water were extracted from specimens during preservation, the influence of extraction

was considered to be minimal, since biomass was compared under the same conditions.

To obtain the population structure of H. longirostris in each month, the carapace length

composition of the four categories (i.e. gravid females, non-gravid females, males, and

juveniles) was accumulated monthly in proportion to density at each station.

3. Results

3.1. Physical environment

The daily freshwater discharge was stable and usually less than 100 m3 s-1 in

fall and winter (October to March), whereas it drastically changed in spring and summer

(April to September), often exceeding 200 m3 s-1 (Fig. 2). In particular, large flood

events (500 m3 s-1) occurred in July, following the minimum discharge (< 20 m3 s-1) in

late June 2005. In 2006, the freshwater discharge was more than 100 m3 s-1 almost all

the time from late June to September, since heavy rainfall events occurred repeatedly.

The means of surface temperature observed at the ten sampling stations on each

sampling date increased from ca. 10 to 30 ºC (spring to summer), before decreasing (fall

to winter: Fig. 2). In comparison with seasonal changes, temperature had minimal

differences among stations in a specific month of each respective year (range 0.7-3.8

ºC). The highest temperature (29.4 ºC) was recorded on 24 July 2005, whereas

temperature was not so high (22.3 ºC) on 25 July 2006 when sampling was conducted

during repetitive flood events.

The salinity profiles along the estuary indicated strong vertical mixing through

2005, with the brackish water front of salinity 1 being located at R5 (17 km upstream

from the river mouth; Fig. 3). Although the downstream movement and partial

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stratification of the front were observed during periods of large freshwater discharges

from June to September in 2006, the front was also located close to R5 with strong

vertical mixing in the other months in 2006 (Fig. 3).

The ETM was usually found close to the brackish water front of salinity 1, with

turbidities more than 100 NTU even in the surface water (Fig. 3). Turbidity was

relatively low from April to August in 2006, being less than 100 NTU through the

estuary in June 2006. Given that ring-net hauls were usually conducted at the surface

of well-mixed water columns (especially in and close to the ETM), surface salinity was

used as an approximate criterion for ambient salinity in the further analyses.

3.2. Species composition and horizontal distribution of mysids

Five mysid species, Hyperacanthomysis longirostris, Neomysis awatschensis,

Neomysis japonica, Orientomysis aspera (Fukuoka and Murano, 2005), and

Rhopalophthalmus orientalis, occurred in the estuary, showing respective patterns of

horizontal distribution in relation to salinity (Fig. 4). H. longirostris was

overwhelmingly dominant (up to 100%) in the upper estuary in 2005 and the second

half of 2006, although its dominance was observed only close to the brackish water

front of salinity 1 in the first half of 2006. N. awatschensis was occasionally dominant

at the uppermost stations (R6 and R7, salinity < 1), whereas N. japonica, O. aspera, and

R. orientalis were often dominant at lower stations, never found upstream from the front

of salinity 1. Among the latter three species, O. aspera and R. orientalis tended to

occur more downstream than N. japonica.

The density of the dominant mysid Hyperacanthomysis longirostris is

separately presented from the other mysids (Fig. 5). The highest density of H.

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longirostris in a specific month of each respective year was usually observed between

salinity 1 and 10, being considerably shifted downstream owing to repetitive flood

events in the summer of 2006. H. longirostris was abundantly (> 10 ind. m-3) collected

at a station in the estuary in May, June, and October to December 2005, and May, July,

October, and November 2006, with the maximum of 666.0 ind. m-3 at R5 on 24 June

2005. Generally, density was low in midsummer (August) and winter to spring

(January to April). In comparison with H. longirostris, the other mysids were less

abundant throughout the estuary all the year around except at lower stations in March

and April 2006.

The relationship between carapace length and dry weight of

Hyperacanthomysis longirostris was well represented by equation (1) (a = 0.035, b =

3.530, N = 77, R2 = 0.952). Spatial and temporal changes in biomass of H. longirostris

closely coincided with those in density (Fig. 5). Biomass was relatively large (> 5 mg

m-3) at a station in the estuary in May, June, October to December 2005, and May, July,

and October 2006, being largest at R4 on 23 May and 24 June 2005 (164.9 and 168.3

mg m-3, respectively).

3.3. Life-cycle stage, sex, and size of H. longirostris

The composition of life-cycle stage, sex, and size of Hyperacanthomysis

longirostris was compared among stations in each month of the respective years (Fig. 6).

Larger individuals, which for the most part consisted of gravid females, were always

distributed at upper stations, gradually decreasing in frequency downstream. In

contrast, larger juveniles were found at lower stations, whereas smaller juveniles were

usually dominant at or close to the peak density of H. longirostris on each sampling date.

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A typical pattern of size differences was observed on 23 May 2005; the mean ± standard

deviation of carapace length of juveniles being 1.1 ± 0.3 (number of specimens

analyzed; N = 26), 1.0 ± 0.3 (N = 75), 1.4 ± 0.3 (N = 58), 1.8 ± 0.3 (N = 22), 1.9 ± 0.3

(N = 25), and 1.9 mm (N =1) from R5 to E1, whereas that of the other individuals being

2.8 ± 0.3 (N = 99), 2.6 ± 0.4 (N = 72), 2.5 ± 0.4 (N = 45), 2.0 ± 0.4 (N = 52), 2.2 ± 0.3

(N = 82), 2.2 ± 0.3 (N = 80), and 2.3 ± 0.3 mm (N = 7) from R6 to E1. Similar

patterns clearly occurred along the estuary in months when H. longirostris was abundant,

being found at least partially in the other months of both years (even in July 2006 when

the largest freshwater discharge was recorded).

The population structure of Hyperacanthomysis longirostris changed

seasonally with a little difference between years (Fig. 7). The smallest juveniles

(carapace length < 1.0 mm) were sampled throughout the year except in midsummer

(July and August) and midwinter (January and February). Males and non-gravid

females were sampled every month, whereas gravid females almost disappeared from

the samples from December to February. In addition, gravid females were very scarce

or absent in April and June 2006, respectively. The monthly means of carapace length

(excluding juveniles) decreased from May to July (2.5 to 2.1 mm), increasing gradually

from July to January (2.1 to 3.5 mm), before reaching a maximum size from January to

March (ca. 3.5 mm). In comparison with 2005, juveniles were overwhelmingly

dominant in June, while almost absent in August 2006.

As for the vertical distribution, Hyperacanthomysis longirostris was always

collected in the surface between 2.5 h before and 3.5 h after the high tide, being more

abundant than in the bottom (Fig. 8). The other mysid species were seldom collected

on both 23 and 24 October 2006. The water mass that we tracked on 24 October

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corresponded exactly with the ETM, as was indicated by low salinities (0.2 to 2.5) and

high turbidities (256 to >1000 NTU) observed. The current velocity ranged from 13 to

104 cm s-1, showing a tendency to be faster in the surface than in the bottom during both

flood and ebb tides. Although females and males of H. longirostris were abundantly

(> 5 inds. m-3) collected mainly in the surface water through the series of sampling,

juveniles were abundantly (> 5 inds. m-3) collected only at lower stations (R2 and R3,

Fig. 8).

4. Discussion

4.1. Inter-specific distribution of mysids

We conducted monthly sampling under a regular condition (around high tide in

the morning during spring tides) each month over a 20 month period, making it possible

to analyze seasonal and annual changes. A large number of specimens were

successively collected along the Chikugo River estuary, although the methods did not

allow sampling of mysids over the entire water column. The lack of information about

the bottom distribution of mysids proved to be not critical, since mysids were collected

in the surface more abundantly than in the bottom (Fig. 8) at least in the ETM around

high tide under normal freshwater discharge. The distribution of mysids is likely to be

mixed vertically by the strong tidal currents particularly during spring tides. In

addition, the estuary is relatively shallow and extremely turbid (depth mostly < 6 m,

turbidity often > 100 NTU; Fig. 3), conditions which possibly minimize the

concentration of mysids near the bottom. Consequently, in the following sections, we

confidently discuss the horizontal distribution and population dynamics of the dominant

mysid in relation to the physical conditions along the estuary.

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Through the course of the sampling period, Hyperacanthomysis longirostris

was dominant close to the brackish water front of salinity 1 in the Chikugo River

estuary, the distribution of which usually corresponded with the ETM (Figs. 3, 5).

Among the other mysids, Neomysis japonica has been reported to be one of the most

dominant estuarine mysids not only in the Ariake Sea (Ikematsu, 1963) but also in the

Pacific coasts of Japan (Yamada et al., 1994; Chihara and Murano, 1997). Although N.

japonica was often dominant in the lower Chikugo River estuary (salinity > 10), the

density of N. japonica did not exceed that of H. longirostris except in spring (Figs. 4, 5).

Orientomysis aspera and Rhopalophthalmus orientalis sometimes occurred more

downstream than N. japonica (Fig. 4), having a preference for higher salinities. In

contrast, Neomysis awatschensis was sometimes dominant at lower salinities in the

upper estuary (salinity < 1; Fig. 4). These species are likely to have different salinity

preferences, although they are generally considered as estuarine and coastal species

(Yamada et al., 1994; Chihara and Murano, 1997).

As for well-known estuarine mysids in the world (e.g. Neomysis americana,

Hulburt, 1957; Laprise and Dodson, 1994; Neomysis integer, Cattrijsse et al., 1994;

Mees et al., 1994; Mesopodopsis slabberi, Moffat and Jones, 1993; Cattrijsse et al.,

1994), salinity is considered as the primary determinant of horizontal distribution,

although turbidity is not always considered as an important factor. However, estuarine

mysids probably include several groups of mysids which have different salinity

preferences, namely different levels of dependence on estuarine environments (e.g. low

salinity, high turbidity, and seaward residual current). In the present study,

Hyperacanthomysis longirostris was solely distributed in a close relation to the ETM

(salinity 1-10), which indicates that H. longirostris is a true estuarine species that is

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totally dependent on estuarine environments. In addition, H. longirostris is a

semi-endemic species in the Ariake Sea (Chihara and Murano, 1997), probably having

been isolated from the original population on the Eurasian continent during geological

history (a continental relict; Sato and Takita, 2000). There are no other estuaries with

well-developed ETM in Japan other than the Ariake Sea, which might account for the

restricted geographic distribution of H. longirostris in Japan.

4.2. Intra-specific distribution of H. longirostris

Freshwater discharge, which is considered as one of the most important

determinants in estuarine environments, is stable in fall and winter, whereas it

drastically fluctuates in spring and summer in the Chikugo River estuary every year.

During the last ten years (1997-2006), freshwater discharge was the second smallest in

2005 (daily mean = 91 m3 s-1) in marked contrast to the second largest in 2006 (150 m3

s-1). As a result, lower temperatures, salinities, and turbidities were observed along the

estuary in the spring and summer of 2006 (Figs. 2, 3). Consequently, it is possible to

reveal both seasonal and annual changes in horizontal distribution and population

dynamics of Hyperacanthomysis longirostris by examination of similarities and

differences between the contrasting years.

The horizontal distribution of Hyperacanthomysis longirostris was different

between sexes and life-cycle stages (Fig. 6), the pattern of which was usually found

despite the fluctuation of physical conditions. Females, particularly gravid females,

were distributed upstream from the center of distribution where small juveniles were

overwhelmingly dominant. In addition, juveniles were larger at lower stations,

whereas the other individuals were larger at upper stations. Similar patterns of

16

distribution have been reported at least partially for several estuarine mysids (Hulburt,

1957; Mauchline, 1970; Orsi, 1986; Hough and Naylor, 1992; Moffat and Jones, 1993;

Baldό et al., 2001). These authors, however, tended to consider the intra-specific

distribution pattern as an incidental result of tidal and/or circulation currents rather than

as a significant mechanism of population maintenance, partly because the pattern was

not so clear in the previous studies compared to the present study.

For an understanding of the intra-specific distribution pattern, we hypothesize a

possible system of population maintenance within an estuary: gravid females release

juveniles in the upper estuary, juveniles grow during downstream transport, young

males and females mature during the upstream migration. Although this hypothesis

comprehensively connected the intra-specific distribution pattern with population

maintenance in an estuary, several points remain to be tested. The first question is

about the consistency in time scales between the growth period of mysids and the speed

of downstream transport. The speed of the residual current in the Chikugo River

estuary is estimated at 9.6 km day-1 (11.1 cm s-1), as the river is approximately 300 m in

width and 3 m in depth with a freshwater discharge of 100 m3 s-1 on average. On the

other hand, rearing experiments have shown that the period for sexual differentiation

depends largely on temperature, being shorter at higher temperatures: 6-22 days at

10-25 ºC for Orientomysis robusta (Sudo, 2003; Fukuoka and Murano, 2005) and 14-42

days at 8-25 ºC for Neomysis integer (Fockedey et al., 2005). Assuming that juvenile

Hyperacanthomysis longirostris has been transported over a 20 km range along the

estuary (R6 to R1; Figs. 1, 6) until sexual differentiation, the speed of downstream

transport is estimated at 0.5-3.3 km day-1 (0.6-3.8 cm s-1), being much slower than the

residual current. The result of the test calculation strongly suggests that juveniles have

17

to minimize downstream transport to develop within the estuary. The second question

is about the mechanism of upstream migration, since the swimming ability of mysids

was lower than 10 cm s-1 even in the adult stage (N. integer; Roast et al., 1998).

Selective transport in strong tidal currents rather than estuarine circulation is most likely

to occur in the macrotidal Chikugo River estuary, as was suggested for several estuarine

mysids (Orsi, 1986; Hough and Naylor, 1992; Kimmerer et al., 1998). In the present

study, females and males of H. longirostris were observed to move upstream with the

ETM during the flood tide, while juveniles seemed to remain downstream (Fig. 8). To

understand the processes controlling the downstream transport as well as upstream

migration, it is, however, necessary to investigate the horizontal and vertical distribution

of mysids and currents at least over a tidal cycle.

The optimal salinity ranges for estuarine mysids are known to be different

between life-cycle stages (Greenwood et al., 1989; Fockedey et al., 2005, 2006). In

the case of Neomysis integer, embryos survived and developed well at salinities of

14-17, whereas juveniles matured only at salinities of 5-15 (Fockedey et al., 2005,

2006). In this context, the intra-specific distribution pattern has to be considered in

relation to the optimal salinity ranges of the respective life-cycle stages of

Hyperacanthomysis longirostris. Although there is need for behavioral and

physiological information about H. longirostris, the intra-specific distribution pattern

would contribute to the population maintenance within the estuary probably through the

hypothesized system.

4.3. Population dynamics of H. longirostris

The density and biomass of Hyperacanthomysis longirostris were relatively

18

high at intermediate temperatures, being low at the lowest and highest temperatures

(Figs. 2, 5). Although either gravid females or juveniles were found in most months,

both were almost absent in January and February (Fig. 7), which indicates that

reproduction was interrupted in midwinter. Minimal growth and reproduction at the

lowest temperatures (ca. 10 ºC) were responsible for the low density and biomasses in

the overwintering generation, as was reported for mysid species in temperate coasts and

estuaries (reviewed in Mauchline, 1980). In midsummer, the density and biomass of H.

longirostris were also low in July, August 2005, and August 2006 when the three

highest temperatures were recorded, whereas they were relatively high in July 2006

when temperature dropped owing to repetitive flood events (Figs. 2, 5). Rearing

experiments of temperate mysids revealed that the mortality of juveniles was higher at

25 ºC than at lower temperatures (Sudo, 2003; Fockedey et al., 2005). In addition,

these experiments showed the influence of temperature on growth, sexual maturity, and

reproduction as well as mortality. Small juveniles were almost absent in July 2005 and

August 2006 despite high proportions of gravid females (Fig. 7), which possibly

suggests high mortalities of juveniles at the highest temperatures (ca. 30 ºC). In

summary, it is likely that recurring recruitments of juveniles support high density and

biomass of H. longirostris at intermediate temperatures (ca. 15-25 ºC) in late spring and

fall, whereas reproduction is interrupted or diminished at the extreme temperatures in

midwinter and midsummer. Although it is necessary to conduct more frequent

sampling to know the exact number of generations, on the basis of monthly changes in

carapace length composition (Fig. 7), H. longirostris has at least one generation other

than the overwintering generation which lives at most from November to May.

Freshwater discharge had various complex effects on the population dynamics

19

of Hyperacanthomysis longirostris, although it clearly shifted the horizontal distribution

of H. longirostris (Fig. 5) as well as salinity and turbidity (Fig. 3). The most notable

difference of density and biomass between years was found in June (Fig. 5). Density

and biomass were the largest during the sampling period under the smallest freshwater

discharge (< 20 m3 s-1) in June 2005, in marked contrast to one of the lowest density and

biomass under repetitive flood events (> 1000 m3 s-1) in June 2006. The large

freshwater discharge not only shifted the salinity front downstream but also toned down

the ETM in June 2006 (Fig. 3), conditions of which may have reduced the population of

H. longirostris. The population was, however, not always diminished by flood events,

since relatively high density and biomass were observed in a turbid water similar to the

ETM offshore from the river mouth in July 2006 (Figs. 3, 5). The ETM was reported

to be especially abundant in copepods and detritus in the Chikugo River estuary (Shoji

et al., 2006; Islam et al., 2006a; Suzuki et al., 2007), potentially providing abundant

foods for mysids (Mauchline, 1980; Takahashi, 2004). There is a possibility that

environmental conditions, including temperature, salinity, turbidity, and food, would be

temporarily improved for H. longirostris all over the innermost Ariake Sea owing to

repetitive flood events in July 2006. As for the true estuarine copepods Sinocalanus

sinensis and Pseudodiaptomus inopinus in the Chikugo River estuary, Ueda et al. (2004)

inferred from high frequency sampling during flood events that adult copepods could

survive just above the bottom and that they could replace the loss of population through

reproduction. H. longirostris might also have a specific strategy to survive flood

events in addition to the hypothesized system to maintain the population under ordinary

conditions.

20

5. Conclusions

The present study demonstrated the horizontal distribution and population

dynamics of the dominant and semi-endemic mysid Hyperacanthomysis longirostris

under the fluctuation of physical conditions along the macrotidal Chikugo River estuary.

As a next step, it is necessary to investigate the population dynamics of H. longirostris

in relation to biological conditions (e.g. food, predator, and competitor) as well as

physical conditions. The biomass of H. longirostris was often as large as the copepod

biomass (up to 100 mg m-3 on average in the Chikugo River estuary, Islam et al., 2006a).

Ecological studies on H. longirostris will lead to a better understanding of the food web

in the ETM, which has been considered as an important nursery for both semi-endemic

and non-semi-endemic fish species in the Ariake Sea (Hibino et al., 1999; Shoji et al.,

2006; Islam et al., 2006a, 2006b; Suzuki et al., 2008a, 2008b).

Acknowledgements

We wish to express our gratitude to Mr. S. Koga of the Okinohata Fisheries

Cooperative Association and Mr. T. Tsukamoto of the Shimochikugo Fisheries

Cooperative Association, for their assistance with the sample collection. We are also

grateful to Mr. S. Akiyama of Field Science Education and Research Center, Kyoto

University, for his valuable information concerning mysid ecology. Dr. T. Wada of

Fukushima Prefectural Fisheries Experimental Station and other graduate students in

our laboratory also assisted with sampling in the field. The present study was

supported in part by Grants-In-Aid from the Ministry of Education, Culture, Sports and

Science.

21

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27

Figure captions

Fig.1. Study area and sampling stations (open circles) along the Chikugo River estuary

of the Ariake Sea, Kyushu Island, Japan. The observatory for freshwater discharge

is represented by a closed circle.

Fig. 2. Daily freshwater discharge (solid line) and monthly temperature (open

diamonds) in the Chikugo River estuary from May 2005 to December 2006.

Monthly temperature is the mean of surface temperature observed at the ten sampling

stations on each sampling date.

Fig. 3. Profiles of salinity and turbidity along the Chikugo River estuary from May 2005

to December 2006. Closed triangles under the horizontal axes represent the

locations of the ten sampling stations.

Fig. 4. Numerical composition of the mysid species collected at the ten sampling

stations along the Chikugo River estuary from May 2005 to December 2006. Solid

and dashed vertical lines represent surface salinity 1 and 10, respectively.

Fig. 5. Density (closed circles) and biomass (dashed lines) of the dominant mysid

Hyperacanthomysis longirostris collected along the Chikugo River estuary from May

2005 to December 2006. Total density of the other mysid species is represented by

open triangles. Solid and dashed vertical lines represent surface salinity 1 and 10,

respectively.

Fig. 6. Carapace length composition of the dominant mysid Hyperacanthomysis

longirostris collected at each sampling station in the Chikugo River estuary from

May to December in 2005 (a) and 2006 (b). Sampling stations are arranged from

the uppermost R7 (left) to the lowermost E3 (right). Four categories of the mysid

(i.e. gravid females, non-gravid females, males, and juveniles) are represented by

28

different patterns in bar graphs. Closed triangles represent stations where the

highest density was observed on each sampling date. N, number of specimens

analyzed.

Fig. 7. Monthly changes in population structure of the dominant mysid

Hyperacanthomysis longirostris in the Chikugo River estuary from May 2005 to

December 2006. Four categories of the mysid (i.e. gravid females, non-gravid

females, males, and juveniles) are represented by different patterns in bar graphs.

Closed triangles represent the means of carapace length in each month (excluding

juveniles).

Fig. 8. Hourly changes in vertical distribution of Hyperacanthomysis longirostris

observed in the estuarine turbidity maximum of the Chikugo River estuary around

high tide on 24 October 2006. All individuals (top) consist of females and males

(middle), and juveniles (bottom). Vertical lines represent the time of high tide.

Fig. 1

130° 131°E

33°N

R6R5

R4

R3

R2

R1

E1

E2

E3

Chikugo River

Ariake Sea

10 km0

32°

KyushuIsland R7

1200

1000

800

600

0

30

20

10

0M J J A S O N D J F M A M J J A S O N D

Fres

hwat

er d

isch

arge

(m3

s-1 )

Temperature (ºC

)

1768

400

200

1596 2053 22431507

Fig. 2

20062005

Turbidity not measured

Turbidity < 100 NTU

0 100 300 500 700 900

Dep

th (m

)

Salinity 1 10 20

Turbidity (NTU)

0

4

4

0

4

0

0

4

0

4

0

4

0

4

0

4

8

0

4

4

0

4

0

0

4

0

4

0

4

0

4

0

4

8

0

4

0

4

0

4

0

4Turbidity not measured

20 10 0 10 20 10 0 10

0

23 May 2005

24 Jun

24 Jul

23 Aug

19 Sep

17 Oct

15 Nov

15 Dec

15 Jan 2006

14 Feb

16 Mar

14 Apr

27 Apr

30 Jun

25 Jul

24 Aug

22 Sep

23 Oct

21 Nov

22 Dec

Salinity 1 10 20

Distance from the river mouth (km)

Dep

th (m

)

Fig. 3

R7 R1 E1 E2 E3R6 R5 R4 R3 R2

Neomysis awatschensisHyperacanthomysis longirostrisNeomysis japonicaOrientomysis asperaRhopalophthalmus orientalisUnidentified

23May2005

24Jul

23Aug

19Sep

17Oct

15Nov

15Dec

27May

23Oct

21Nov

22Dec

15Jan2006

16Mar

14Apr

14Feb

25Jul

24Aug

22Sep

30Jun

Salinity 1 10

24Jun

R7 R1 E1 E2 E3R6 R5 R4 R3 R2

Salinity 1 10N

umer

ical

com

posi

tion

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

0

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

1.0

0.5

0

Num

eric

al c

ompo

sitio

n

Sampling station

Fig. 4

Salinity 1 10

20 10 0 10 20 10 0 10

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

0.1

10

1000

1000

1000

1000

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

10

1000

0.1

23May2005

24Jul

23Aug

19Sep

17Oct

15Nov

15Dec

24Jun

27May

25Jul

24Aug

22Sep

23Oct

21Nov

22Dec

30Jun

15Jan2006

16Mar

14Apr

14Feb

Hyperacanthomysis longirostrisDensity + 0.1 (ind. m-3)Biomass + 0.1 (mg m-3)

Other species Density + 0.1 (ind. m-3)

Salinity 1 10

Distance from the river mouth (km)

Den

sity

(ind

. m-3

) / B

iom

ass

(mg

m-3

)

Den

sity

(ind

. m-3

) / B

iom

ass

(mg

m-3

)

Fig. 5

10

10

10

R6 R5 R4 R3 R2 R1 E1N=99 N=98 N=120 N=110 N=104 N=105 N=8

E2

N=100 N=102 N=89 N=32 N=101 N=100 N=29

N=100 N=100 N=42

N=8 N=60 N=216

012345

N=10 N=100 N=100 N=140 N=100 N=100 N=32 N=21

N=15 N=100 N=100 N=100

0 0 0 00.3 0.3 0.3 0.3

0

0 0 00.3 0.3 0.3

0.3 0 0 00.3 0.3 0.3

0 0.300.3 0.30 0.301234

01234

1234

012345

01234

N=28N=100 N=99 N=100

1234

N=100 N=100 N=100 N=100 N=100 N=100 N=72

01234

0 0.3

R7

19Sep

17Oct

Gravid femaleNon-gravid femaleMaleJuvenile

0.51

0.56

0.53

Car

apac

e le

ngth

(mm

)

Frequency

Fig. 6a E3

23May2005

24Jun

24Jul

23Aug

15Nov

15Dec

Fig. 6b R7N=15N=86 N=200

N=15 N=4

N=11N=100 N=99

N=50N=100 0 0.30 0.3 0.3

0 00.3 0.3

0 0.30 0.3

N=78 N=200 N=100 N=100 N=100 N=100

N=100 N=100 N=100 N=100 0.3 0.30

0 0 0.3 0.3

0

0 00.3 0.3 0.3 0

0.30

R4 R3 R2 R1 E1

012345

1234

0

0

12345

12345

12345

N=5N=100 N=100 N=201 N=100

01234

01234

N=17N=100 N=100 N=201 N=99

01234

Gravid femaleNon-gravid femaleMaleJuvenile

Car

apac

e le

th (m

m)

Frequency

R6 R5 E2 E3

27May2006

30Jun

25Jul

24Aug

ng

22Sep

23Oct

21Nov

22Dec

Fig. 7

23May2005

24Jul

23Aug

19Sep

17Oct

15Nov

15Dec

24Jun

27May

25Jul

24Aug

23Oct

21Nov

22Dec

30Jun

15Jan2006

16Mar

14Apr

14Feb

22Sep

0

1

2

3

4

5

0

1

2

3

4

0

1

2

3

4

0

1

2

3

4

Gravid femaleNon-gravid femaleMaleJuvenileMean length (excluding juveniles)

0 0.3 0 0.3

0 0.30 0.30 0.30.3

Frequency

Car

apac

e le

ngth

(mm

)

Fig. 8

All individuals

Females and males

Juveniles

Dep

th (m

)

0

2

0

2

0

2

4

4

4

2.5(R2)

1.5(R3)

0.5(R4)

0.5(R5)

1.5(R4)

2.5(R3)

3.5(R2)

0 5 10 20 30 40 Density (ind. m-3)

Hours before/after high tide(Sampling station)